What if I told you that the best gas mileage calculator is likely the cheapest one around? A simple notebook and pencil kept in the drivers side door is one of the simplest ways to track your gas mileage, even in this world of high-tech, solid state electronics.
With each gas fill up, simply record the date, the number of gallons used to fill the tank, the current odometer reading, and optionally the cost of the gas (either per gallon or for the entire tank)
The only "rule" when keeping track of gas mileage with this method is that you must fill the tank every time you stop to get gas. Why is it important to fill the tank? Practical reasons would tell you that you'll be able to drive further without having to refill! But when using your paper & pencil gas mileage calculator, the only way you can tell how many miles were traveled per gallon of gas is to know how many gallons of gas were used since you last topped off the gas tank.
If you start with a tank that is not quite full, drive around for 100 miles and then try to fill the tank again, how many gallons were used to drive that 100 miles? You'd never know unless you started the trip with a full tank, and ended the trip with a full tank. Then you know exactly how many gallons were used. Hint: it's the number of gallons needed to fill up the tank after the trip.
Once you've been keeping track of your fill ups this way, simply take your handy laptop and notebook into a coffee shop (that sells fair-trade coffee, of course) and enter your numbers into an excell spreadsheet.
Your column headings would start this way:
B Gallons (used to fill tank)
C Odometer Reading
You'll need at least two rows of information to calculate your gas mileage. but if you transfer your data monthly, you'll have plenty of information to let Excel cruch.
You'll need to set up some calculated fields like this:
D Miles per tank = Odometer Reading (current minus previous)
E Mileage (Miles per Gallon) = D (miles per tank) divided by B (Gallons per tank)
When you dived miles per tank by gallons per tank, the "tank" units cancel out,and you are left with miles per gallon, which is your gas mileage.
It's pretty simple math, and tracking your gas mileage can help you know when your car needs serviced or a tune-up.